1. Field of Invention
The present invention relates to a digital image processing apparatus, and more particularly to compression of the image data.
2. Description of Related Art
In recent years, digital photocopiers which generate a hard copy of a manuscript by reading the manuscript image using an image input apparatus, for example a scanner, digitally processing the input image data, and outputting the digitally processed data to an image output apparatus, for example a printer, have come into widespread use.
In the digital photocopier, it is essential to store a plurality of the image data in the photocopier, have an electronic sorter function which sorts the data (for example, manuscript, files, and edits pages), and to have an electronic RDH function. This is accomplished by equipping the copier with an interval data storage apparatus, for example, random access memory or a hard disk, storing the image data therein and outputting the data as necessary. In order to store a large amount of image data, it is necessary to increase the storage capacity of the storage apparatus, but this results in an increase in the size and cost of the apparatus itself. In order to avoid this, a method of storage that compresses the image data has been proposed. By compressing the image data, it is possible to store a large amount of the image data using smaller-capacity storage apparatus.
Furthermore, the image output apparatus maybe a laser printer. In general, a page descriptive language is used for controlling the method of image output to the printer. A host computer, to which the printer is connected, does not transfer to the printer the output content itself as a bitmap image (raster image) but rather the content of the page descriptive language describing the character and image information of that output content. The printer receives the page descriptive language, internally interprets the language content, render the image data of the page as a bitmap image (raster image), and outputs the image by transferring the image to the paper.
Therefore it is essential to have sufficient printer memory to maintain the bitmap image which renders the image data and a function to interpret the content of the page descriptive language. For example, memory would require 32 megabytes if the output resolution is 400 dpi and the output gradation degree is 256 steps, in the case of a monochrome printer which outputs A3 size paper.
In the case of a color printer, the output of YMCK 4 colors is needed, so the memory capacity becomes four times larger, to 128 megabytes. Obtaining such large memory capacity necessarily increases the size, cost, etc., of the printer itself. To avoid this increase in memory capacity, just like the case of the digital photocopier, the image data can be stored in the compressed format. In doing so, large amounts of image data can be stored using smaller memory capacity.
In the case of compressing the image data and reducing the data capacity, reducing the gradation degree of the image repeatedly and storing the image in a binary state can be considered, however, the quality of the image output which can be eventually obtained deteriorates when the gradation degree is reduced. Thus, in order to store images of high quality, it is better to store the image in a multi-value state rather than in a binary state.
In order to compress this multi-value image data, many methods exist.
With manuscripts output by digital photocopier or printer, it is often the case that text area and photo area co-exist on a single sheet. Additionally, with printer output images, there are many manuscripts that mix both images created by computer, or so-called computer graphics (CG), and photos and other such read-in images scanned from a scanner.
These CG and scanned images each have different image characteristics. For instance, CG image areas include many flat areas in which the pixel values fluctuate either uniformly or not at all. Furthermore, even within the CG image area, the character area in which only the white and black binary exists and the gradation area in which the original element value drastically changes also exists. In contrast, in many instances the scanned images area includes noise picked up during reading by a scanner, causing minute fluctuations in the pixel value.
Additionally, the CG and scanned image boundaries have different image characteristics. For this reason, effectively compressing mixed image data with high quality requires the optimum compression process for each set of image quality characteristics. In order to meet this need for image data mixing small areas having varied image characteristics it is necessary to select the optimum compression process for each area depending on the image characteristics.
An adaptive image compression method has often been proposed. The CG area, often including images requiring a high degree of resolution such as characters, line drawings, and the like, a compression method in which the resolution data does not deteriorate is preferred. For example, reversible compression methods such as MMR, LZW, JBIG and the like and block compression methods such as BTC and the like, in which the gradation data deteriorates but the resolution data does not, are appropriate.
Scanned image areas often include images requiring gradation data more than resolution data, for example photos, natural images, and the like, where a compression method in which the gradation does not deteriorate is preferred. In the case of applying the reversible compression method in which the image does not deteriorate after decompression to the scanned image area, the pixel values severely change in this area and entropy is high, so it is not possible to effectively compress data by the reversible compression method.
Therefore, the non-reversible compression method is applied for the scanned images area. Among the non-reversible compression methods, a method which is able to maintain the gradation data after decompression is used. For instance, there is the Adaptive Discrete Cosine Transform (ADCT) method or the like, typified by the JPEG baseline, which is used as the standard encoding method for color facsimile.
One object of the invention is to select image compression means resulting in a reduction in required memory in an image processing device.
Another object of the invention is to compare the compressed image data to a target and reselect and recompress the data until the target is satisfied.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations pointed out in the appended claims.